Metal Finishing Guide Book

2013

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troubleshooting, testing, & analysis CHOOSING AN ACCELERATED CORROSION TEST FRANK ALTMAYER SCIENTIFIC CONTROL LABORATORIES INC., CHICAGO; WWW.SCLWEB.COM Accelerated corrosion tests are typically used to determine if a coating/substrate combination has been produced to yield a satisfactory service based on historical data from previous testing and field exposures of similar coating/substrate combinations. The intent is to find out, in a relatively short amount of time, what the appearance or performance of the product will be after several years of service. Real-life exposures are complicated events that may involve several factors including geometric configuration, porosity/adherence of corrosion product, soiling, abrasion, frequency of cleaning, cleaning procedures, cleaning chemicals, sun exposure, and temperature variations. Because of this, it is critical that the accelerated test chosen simulates "real-life" corrosion mechanisms as much as possible. The following guidelines were prepared to assist in choosing the best accelerated corrosion test for a given application. CORROSION MECHANISMS Coated metallic products are subjected to two basic corrosion mechanisms during their service life: (1) electrochemical (galvanic) and (2) chemical attack. Electrochemical (Galvanic) Electrochemical corrosion can be caused by dissimilar metals contacting an electrolyte. This is the common "battery" effect. Detrimental galvanic corrosion effects occur when the substrate is electrochemically more active than the protective coating, or when the corrosive environment contains a metal that is less active than the coating and substrate. The electrolyte (water, salt solution, acid, etc.) must be in contact with both metals for this mechanism to occur. Examples of the beneficial use of this corrosion mechanism include galvanized or electroplated zinc over steel, where the zinc, being electrochemically more active than steel, will corrode in preference to the steel when exposed to a corrosive environment (electrolyte). This protection is extended even if large scratches are present through the zinc and into the steel. Another example is duplex nickel. Nickel containing sulfur (~0.02%) from the addition of brightening agents is electrochemically more active than nickel without sulfur. A two-layer system, consisting of semibright nickel (no sulfur) followed by bright nickel, yields a galvanic couple wherein the bright nickel corrodes in preference to the semibright, providing galvanic protection to the second nickel layer, thereby delaying corrosion of the basis metal (and failure). The electrochemical mechanism can also involve a difference in the quantity of oxygen contacting the surface of the exposed specimen in the presence of an electrolyte. Two dissimilar metals are not required. The area of the specimen that is oxygen deficient becomes anodic to the area that contacts the larger quantity of oxygen. The anodic area dissolves into the electrolyte, leaving a corrosion pit. Hydroxides (alkali) are deposited at the cathodic (oxygen-rich) area. 506

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